Corona discharge air transporting arrangement

ABSTRACT

An arrangement for transporting air with the aid of so-called ion-wind includes at least one corona electrode (K) and at least one target electrode (M) located downstream of the corona electrode at a distance therefrom. The arrangement also includes a direct-current voltage source, the two terminals of which are connected to the corona electrode and the target electrode respectively. The construction of the corona electrode and the voltage of the voltage source are such that a corona discharge generating air ions occurs at the corona electrode. The occurrence of an ion current flowing in a direction upstream from the corona electrode, and thus counter acting the desired direction of air transport, is prevented by effectively screening the corona electrode in a manner such that the strength of any ion current flowing in the upstream direction and the distance through which such an ion current migrates from the corona electrode is practically zero, or in all events much smaller than the product of the ion-current strength and the distance migrated by the ion current in a direction downstream from the corona electrode. The distance from the corona electrode to that part of the target electrode receiving the predominant part of the ion current is at least 50 mm, and preferably at least 80 mm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an arrangement for transporting airwith the aid of so-called ion-wind or corona-wind, the arrangement beingof the kind set forth in the preamble of claim 1.

2. The Prior Art

The arrangement has been developed primarily for use in conjunction withair purifying devices, such as electrostatic precipitators for example,and air processing systems, such as ventilation systems andair-conditioning systems, for example, although the invention can alsobe used to advantage in many other connections where air is required tobe transported, such as when cooling electrical apparatus or electricalequipment, and in conjunction with heating devices, such as electrichot-air blaze.

Today, air is transported in the aforesaid apparatus, systems etc.almost exclusively with the aid of mechanical fans of mutually differentdesign. Such mechanical fans and associated drive motors are relativelyexpensive, in addition to being heavy and requiring a considerableamount of space. They also have a relatively high energy requirement,and are consequently expensive to run. In operation the fans alsogenerate a considerable amount of noise, which is highly troublesome inmany areas in which such fans or blowers are used, for example indwelling places and in certain working locations.

It is known that the transportation of air can be achieved, inprinciple, with the aid of so-called ion-wind or corona-wind. Anion-wind is created when a corona electrode and a target electrode arearranged at a distance from one another and each connected to arespective terminal of a direct-current voltage source, thecorona-electrode design and the voltage of the direct-current voltagesource being such as to cause a corona discharge at the coronaelectrode. This corona discharge results in ionization of the air, withthe ions having the same polarity as the polarity of the corona element,and possibly also electrically charged so-called aerosols, i.e. solidparticles or liquid particles present in the air and becomingelectrically charged upon collision with the electrically charged airions. The air ions move rapidly, under the influence of the electricfield, from the corona electrode to the target electrode, where theyrelinquish their electric charge and return to electrically neutral airmolecules. During their passage between the electrodes, the air ions areconstantly in collision with the electrically neutral air molecules,whereby the electrostatic forces are also transferred to these latterair molecules, which are thus drawn with the air ions in a directionfrom the corona electrode to the target electrode, thereby causing airto be transported in the form of a so-called ion-wind or corona-wind.

Arrangements for transporting air with the aid of ion-winds are known tothe art, and examples of such apparatus are described and illustrated,inter alia, in DE-OS No. 2 854 716, DE-OS No. 2 538 959, GB-A No. 2 112582, EP-Al No. 29 421 and U.S. Pat. No. 4,380,720. These prior artair-transporting arrangements utilizing ion-wind or corona-wind havebeen found extremely ineffective however, and have not achieved anypractical significance. It would seem that a reason for this is a lackof understanding of the physical mechanisms responsible for the totaltransportation of air through an arrangement of this kind. Consequently,it is not possible with the previously suggested embodiments of ion-windoperated air transporting arrangements to achieve in practice thetransportation of significant quantities of air without needing to raisethe corona current to levels which lie considerably above those levelswhich can be considered acceptable when using such an arrangement inpopulated environments. It is well known, inter alia, from theelectrostatic precipitator field, that an electric corona dischargegenerates chemical compounds, primarily ozone and oxides of nitrogen,which have an irritating effect on human beings, and which can beharmful to the health when present in the air in excessively highconcentrations. In the event of a corona discharge these chemicalcompounds are generated at a rate which is contingent on the magnitudeand polarity of the electric corona current. Consequently, present dayelectrostatic air filters for use in human, or populated, environmentsoperate with a positive corona discharge and a corona current having anamperage which is substantially proportional to the quantity of airpassing through the filter per unit of time in normal operatingconditions. In this respect the corona current is of the order of 40-80μA at an air-throughput of 100 m³ /h, the strength of the current beingadapted to the requirement for an acceptable level of ozone and Noxgeneration. It will be understood that the corona current utilized inair-transporting arrangements which operate with an ion-wind and areused in the presence of people, i.e. human environments, must also berestricted to the aforesaid magnitude. This is not possible to achievewith the prior art air transporting arrangements utilizing ion-wind, dueto the poor efficiency of the arrangements. For example, according toreports, it is possible to achieve with the arrangement proposed inEP-Al No. 29 421 and U.S. Pat. No. 4,380,720 an air throughput of 1 l/swith the aid of a corona power of 1 W at a preferred corona voltage of15 kV. Thus, when converted to an air throughput of 100 m³ /h thisarrangement will consume about 1900 μA, which is roughly thirty timeshigher than the corona-current value acceptable in human environments.

Consequently an object of the present invention is to provide animproved and much more effective air transporting arrangement of thekind mentioned in the introduction, and one which is so efficient as toenable it also to be used in practice in a human environment.

The arrangement according to the invention is based on a more profoundand improved understanding, previously unachieved, of the mechanismsdecisive for the total transportation of air through an arrangement ofthis kind, and has the characterizing features set forth in thefollowing claims.

BRIEF DESCRIPTION OF THE INVENTION

The invention will now be described in more detail with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of ion migration between a coronaelectrode and a target electrode;

FIGS. 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, and 13 illustrate schematically anumber of different embodiments of an arrangement according to theinvention; and

FIG. 8 is a diagram of the corona current as a function of the voltage.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT

There will first be given a synopsis of the fundamental conditionsdeterminative for the transportation of air capable of being obtainedwith the aid of an ion-wind or corona-wind generated between a coronaelectrode and a target electrode arranged axially downstream of thecorona electrode in the desired flow direction. FIG. 1 illustratesschematically a corona electrode K in the form of a thin wire extendingacross the airflow path, e.g. across an airflow duct, and a targetelectrode M which also extends across the airflow path and which isshown schematically and by way of example, in the form of a net or gridstructure which is permeable to the airflow. The target electrode M isplaced downstream of the corona electrode K in the desired direction ofairflow, shown by an arrow w, at an axial distance H from the coronaelectrode K.

As previously mentioned, the corona discharge created at the coronaelectrode gives rise to electrically charged air ions, which migrate ina direction towards the target electrode under the influence of theelectric field present between the corona electrode and the targetelectrode.

The mobility of the ions varies within a wide spectrum, although for thepresent purpose it can be assumed that lightweight ions having themobility

    c=2.5·10.sup.-4 m.sup.2 /Vs

are predominant, and that any electrically charged aerosols present,which are far less mobile than the air ions, only constitute anegligible part of the total charge in the system. It can also beassumed that the air ions constitute a very small fraction of the totalmass of the air within the system, and that the flow rate of the air isat least one power of ten lower than the speed of motion of the airions. Thus, with respect to the migration velocity of the air ions thesurrounding air can be assumed to be stationary.

The migration velocity v of electrically charged air ions in relation tothe surrounding air is proportional to the product of their mobility cand the strength E of the electric field and hence

    v=c·E                                             (1)

It is also assumed that steady state conditions prevail, so that thecharge density in a given part-volume of the system is constant, i.e.that the electrical charge per unit time supplied to the system is equalto that removed from the system. Consequently, the current density inthe air can be expressed as the product of the migration velocity v ofthe charges and the charge density ρ

    i=ρ·v                                         (2)

where i is the current density.

The specific volumetric force in the air is the product of the chargedensity ρ and the electric field strength E, and hence

    f=ρ·E                                         (3)

where f is the driving force per unit volume of air.

When applying the above equations (1), (2) and (3) there is thusobtained

    f=i/c                                                      (4)

i.e. the specific volumetric force can be expressed as the ratio of thecurrent density to the ion mobility.

As illustrated in FIG. 1, we now consider a "current duct", whichconducts an infinitesimally small part dI of the total ion flow Ibetween the two electrodes K and M. The centre line of this current ductis always parallel with the current density vector i and itscross-sectional area dS has a surface normal which is parallel with thecurrent-density vector.

We now consider a volume element

    dV=dS·dl                                          (5)

of this current duct, where dV is an infinitesimal volume and dl is aninfinitesimal length in the direction of the current duct. The forceacting in the direction of the surface normal on each such volumeelement in the current duct becomes

    dF=f·dV=f·dS·dl                 (6)

This volumentric force dF has a component in the direction w of airtransportation and a component at right angles to said direction. It isassumed that when totalled across the whole cross-sectional area of theairflow path or duct in the arrangement these transverse forces willcancel out each other and can therefore be ignored. Consequently, thetotal transportation force in a current duct is ##EQU1## where H is thedistance between the corona electrode K and the target electrode M inthe direction of airflow.

The total transportation force F_(T) in the airflow duct can thus beexpressed as ##EQU2## where S is the total cross-sectional area of theairflow duct and I is the total ion or corona current.

Thus, the average pressure setup can be written as

    Δp=F.sub.T /S=H/c·I/S                       (9)

The transportation force is thus proportional to the product of thetotal ion or corona current I and its migration path H, i.e.proportional to the so-called "current-distance" H.I.

It can be shown that the total air throughput as a result of thispressure setup can be written as ##EQU3## where Q is the air throughput,k is a dimensionless aerodynamic resistance coefficient and λ_(A) is thedensity of the air.

It will be seen from the equation (10) that the magnitude of airtransportation is directly proportional to the square root of theproduct between the total ion or corona current I and its migrationdistance H.

Thus, in order to achieve a high air throughput in the desireddirection, i.e. in a direction away from the corona electrode andtowards the target electrode, it should be endeavoured to attain a highproduct of the ion current and its migration distance in a directiondownstream from the corona electrode, i.e. from the corona electrodetowards the target electrode. An increase in the transporting force, andtherewith in the total air throughput, can be achieved either byincreasing the strength of the total ion current or by increasing thedistance between the corona electrode and the target electrode. Asbeforementioned, when used in a human environment, however, it is notpermissable to increase the strength of the ion or corona current to alevel which exceeds a given maximum in view of the ensuing production ofharmful ozone and oxides of nitrogen (Nox), this production beingprimarily proportional to the corona current. Consequently, the onlyremaining parameter capable of being influenced in this regard is thedistance migrated by the corona current, i.e. the axial distance betweenthe corona electrode and the target electrode. Accordingly, it isproposed in accordance with the invention that the distance between thecorona electrode and the part of the target electrode receiving thepredominant part of the ion current is at shortest 50 mm, and preferablymeasures at least 80 mm.

It will also be seen that when using an air transportation arrangementof the aforedescribed kind, a stream of air ions is also able to migratefrom the corona electrode in an upstream direction, i.e. in a directionopposite to the desired direction of air transportation, if there islocated upstream of the corona electrode an electrically conductiveobject or subject having an electrical potential in relation to thecorona electrode which makes such migration of the air ions possible. Itwill be understood that this greatly reduces the total desiredtransportation of air through the arrangement. To the extent that thispossibility of a stream of ions passing from the corona electrode in anupstream direction therefrom has been taken into account when designingknown air transporting arrangements of the kind discussed here, it wouldappear to have been assumed sufficient to ensure that electricallyconductive objects upstream of the corona electrode are located at aconsiderable distance therefrom and that the flow of ion currentdirected upstream is small. However, since the transportation forcecreated by the ion flow is proportional to the product of the strengthof said flow and the distance travelled thereby, as made evident in theabove equation (9), it will be seen, to the contrary, that even a verysmall stream of ions from the corona electrode in a direction upstreamtherefrom can give rise to a significant transportation force in adirection opposite to the desired direction of air transportation, whenthis upstream directed stream of ions has a long path to travel.

It must be observed in the present context that the term "electricallyconductive" must be interpreted in relation to the extremely smallcurrent strengths prevailing in an arrangement of the present kind,these current strengths normally being of the order of 1 mA/m².Consequently, in the case of an air transporting arrangement of the kindto which the present invention refers, objects which can be consideredto be electrically conductive or which have a surface which can beconsidered as electrically conductive will, in practice, always be foundupstream of the corona electrode. These objects may, for example,comprise grids or net structures or other parts of the arrangementitself located at the inlet to the airflow duct of the arrangement. Evenin the absence of such arrangement components, such objects as wallsurfaces, pieces of equipment or furniture and even people, which arepresent in the area in which the arrangement is placed and located inthe vicinity of the inlet to the airflow duct of the arrangement canserve as electrically conductive surfaces to which a stream of ions canmigrate from the corona electrode upstream in the duct.

This sought for improvement in efficiency, i.e. a high air throughputwith the aid of a corona current limited to an acceptable value, isachieved in the air transporting arrangement according to the invention,partly by locating the target electrode at such a distance from thecorona electrode that the distance from the corona electrode to thatpart of the target electrode receiving the predominant part of the ioncurrent, i.e. the migration distance of the ion current downstream fromthe corona electrode, is at shortest 50 mm, and preferably not shorterthan 80 mm, and partly by ensuring that the product of the ion-currentstrength and the distance migrated by the current in the upstreamdirection away from the corona electrode is practically zero, or in allevents much smaller than the corresponding product of ion-currentstrength and the migration distance of the current in the downstreamdirection, away from the corona electrode. This latter is effected inaccordance with the invention by effectively screening the coronaelectrode in the upstream direction, so that no ion current is able toflow from the corona electrode in the upstream direction, or at least sothat any ion current able to flow in the upstream direction is only verysmall and travels through only a very short distance.

According to one embodiment of the invention, the aforesaid necessaryscreening of the corona electrode in the upstream direction can beachieved by connecting the terminal of the direct current sourceconnected to the corona electrode to a potential which coincidessubstantially with the potential of the immediate surroundings of thearrangement, i.e. in practice is earthed in the same manner as thecasing which houses the arrangement and as the remaining inactive,electrical components. To the extent that it has previously beenproposed in conjunction with air transporting arrangements of this kindto locate the corona electrode at earth potential instead of a highpotential, these two alternatives have previously been considered to beequivalent to one another with respect to the mechanism of airtransportation, and connection of the corona electrode to earthpotential has not been effected in an endeavour to screen the coronaelectrode in the upstream direction.

In many cases, however, it is not desirable to connect the coronaelectrode to earth potential, since for various practical reasons it maybe desired to connect the target electrode to earth potential, or toconnect the corona electrode and the target electrode to oppositepolarities relative to earth, and therewith reduce the need forhigh-voltage insulation. In cases such as these the desired screening ofthe corona electrode in the upstream direction can be achieved, inaccordance with another embodiment of the invention, with the aid of amethod known from other areas of the electrotechnical field, byarranging an electrically conductive screening element upstream of thecorona electrode and giving to said element a potential which coincidessubstantially with the potential of the corona electrode, so that theyform upstream of the corona electrode an equipotential barrier which issubstantially impenetrable to ions flowing in the upstream direction. Tothe extent that the provision of a screen electrode upstream of thecorona electrode and connected to the same potential as said electrodehas been previously proposed in conjunction with air transportingarrangement of the kind in question, such proposals have been made inconjunction with an air transporting arrangement of cascadeconstruction, comprising a plurality of corona-electrode arrays andtarget-electrode arrays arranged in axial sequential relationship in anairflow duct. It has not earlier been understood or perceived thateffective screening of the corona electrode against an ion current inthe upstream direction is, under all circumstances, essential to theefficiency of the air transporting arrangement.

A third and extremely surprising possibility of effecting the necessaryscreening of the corona electrode against an undesirable flow of ions inthe upstream direction resides in extending an airflow duct encompassingthe electrodes of the arrangement through a substantial distanceupstream of the corona electrode, i.e. at the inlet end of the airflowduct, the walls of said duct expediently consisting of a dielectricmaterial, for example a suitable plastics material, in a known andobvious manner. Tests have shown that when operating an air transportingarrangement of the kind in question, there appears on the dielectricwalls of the airflow duct an excess of electric surface charges whichremain all the time the material is subject to the prevailing electricfield. By "excess charges" is meant here electrical charges on thesurface of the dielectric material additional to the surface chargesassumed by the classical understanding of dielectric material of weakelectrical conductivity. It has not been clearly established why theseexcess charges occur on the dielectric walls of the airflow duct,although the phenomenon itself has been established experimentally. Thephenomenon would seem to be related to the phenomena utilized whenmanufacturing dielectric electrets. In this latter case, specialdielectric material is subjected to a combination of highly electricfield and ion currents. Electrical excess charges are therewith boundpermanently in the structure of the material, and are not conducted awaydespite the fact that the material is electrically conductive to acertain degree. Consequently, in conjunction with aforestated phenomenonencountered in air transporting arrangements of the kind in question, itis an obvious assumption to one skilled in this art that the electricalexcess charges on the dielectric walls of the airflow duct are alsobound to the structure of the dielectric material, but only providedthat the material is exposed to the influence of an electric field. Thisphenomenon can be used beneficially to achieve necessary screening ofthe corona electrode in the upstream direction, by extending the airflowduct and its dielectric walls upstream, away from the corona electrode,i.e. at the inlet end of the duct, through a distance such that theexcess charges appearing on the duct walls under the influence of an ioncurrent from the corona electrode immediately after switching on thearrangement, effectively screen the ion cloud present around the coronaelectrode against the possible occurrence of an electric field upstreamof the corona electrode, so as to obtain thereby an effective shieldagainst an upstream-directed ion current from the corona electrode. Itwill be seen that the further the airflow duct is extended upstream ofthe corona electrode, the greater the efficiency of the screen provided.Tests have shown that a satisfactory screening effect can be obtainedwhen the distance through which the airflow duct is extended upstream ofthe corona electrode is at least 1.5 times the distance between thecorona electrode and the target electrode. It will also be seen that thescreening effect becomes more efficient with decreasing widths of theairflow duct, i.e. the smaller the distance between mutually opposingdielectric walls, the greater the efficiency of the screening effectproduced. In the case of an airflow duct of relatively largecross-sectional area, the screening effect can be increasedsubstantially, by dividing the duct into a plurality of mutuallyparallel part-ducts upstream of the corona electrode, with the aid ofelongated partition walls extending parallel with the walls of the duct,for example partition walls in the form of strips or the like ofdielectric material. An arrangement such as this will enable the coronaelectrode to be screened effectively against an ion current in theupstream direction even though the distance to which the airflow duct isextended upstream of the corona electrode is only roughly equal to thedistance between the corona electrode and the target electrode.

Another serious problem encountered with air transporting arrangementsof this kind intended for use in a human environment, is that they mustbe safe to touch in spite of the high voltages used. A touch guard can,of course, be provided with the aid of mechanical means, by providingthe airflow duct surrounding the electrodes of the arrangement withfully impervious walls and fitting the duct with a protective grid atboth its inlet and its outlet end, so that it is impossible to touch thevoltage carrying electrodes of the arrangement, either unintentionallyor intentionally. Such guards, however, present a significant resistanceto flow and therewith seriously impair the transport of air through thearrangement, and therewith its efficiency. It has been found possible inan arrangement according to the invention, however, to provide perfectlysatisfactory safety precautions against contact with the arrangement ina much simpler and more advantageous manner. As described in theaforegoing, an arrangement constructed in accordance with the presentinvention operates with an extremely low corona current, in the order of20-50 μA per 100 m³ /h transported air. This extremely low specificvalue of the corona current is made possible due to the large axialdistance between corona electrode and target electrode, and theeffective screening of the corona electrode in the upstream direction.As a result of this low current consumption, the voltage carryingelectrodes of the arrangement, irrespective of whether it is the coronaelectrode or the target electrode, can be connected to its associatedterminal of the voltage source through an extremely high resistance,without needing to increase the voltage of the voltage source to anunacceptable extent. It has been found that this series resistance canbe readily given, with no difficulty whatsoever, a resistance value ofsuch high magnitude that in the event of the voltage carrying electrodebeing short-circuited directly, the short circuiting current is so lowas to be totally harmless. A limit value of 2 mA is normally set withregard to a harmless short circuiting current from the aspect of bodilycontact with such electrical appliances. If the short circuiting currentis made as low as about 100-300 μA, no unpleasant sensations at all areexperienced when touching the voltage carrying electrode. This canreadily be achieved with an arrangement according to the invention. Ifit is assumed, for example, that the voltage carrying electrode of anarrangement shall have an operating voltage of 20 kV and the coronacurrent is 50 μA, the voltage carrying electrode can be connected to thecorresponding terminal of the voltage source through a resistance of,for example, 150 MΩ, wherewith the voltage source itself must thus havea terminal voltage of 27.5 kV. When the voltage carrying electrode isdirectly shortcircuited, the short circuiting current will therewith besolely about 185 μA, which is of such low magnitude as to cause nodiscomfort, should the short circuit be caused by direct contact withthe electrode. This limitation of the short circuiting current to avalue which causes no discomfort when coming into direct personalcontact with the voltage carrying electrode has been totallyunattainable in practice, however, with the large corona currents, inthe order of 2000 μA, which must, of necessity, be used in prior art airtransporting arrangements operating with an electric ion-wind. Anothersignificant factor of the contact safety-precaution, additional to thelow level of the short circuiting current, is the capacitive dischargecurrent which can occur when an electrode of a given capacitance istouched. In the case of electrodes of such design as to have significantcapacitance, however, the capacitive discharge current can be reduced tofully acceptable levels, by forming these electrodes from a material ofhigh resistivity, in accordance with the invention. This creates noother drawbacks, since the electrodes do not need to be highlyconductive, in view of the low current strengths which can be used inaccordance with the invention while still providing an efficient airtransporting arrangement.

FIG. 2 of the accompanying drawings illustrates schematically and by wayof example the principle construction of a first embodiment of an airtransporting arrangement according to the invention. This arrangementincludes an airflow duct 1 which is made of an electrically insulatingmaterial and through which a flow of air is to be produced in thedirection identified by an arrow 2. Arranged in the airflow duct is acorona electrode K which is permeable to the airflow, while arrangedaxially downstream of the corona electrode is a target electrode M,which is also permeable to the airflow. The corona electrode K comprisesan electrically conductive material, which is preferably ozone andultraviolet resistant, and may be constructed in a number of differentknown ways, to proof an electric field. The corona electrode K of theFIG. 2 embodiment is shown, by way of example, to comprise a thin wireor filament which extends across the airflow duct 1. The coronaelectrode may have many other different forms however. For example, itmay comprise a plurality of thin wires or filaments arranged eitherparallel with one another or in the form of an open mesh grid or net.Instead of using straight, thin wires or filaments, the wires may bewound spirally, or thin strips exhibiting straight, serrated orundulating edge surfaces may be arranged in a similar manner. The coronaelectrode may also comprise one or more needle-like electrode elementsdirected substantially axially in the airflow duct 1. The targetelectrode M comprises an electrically conductive or semi-conductivematerial, or a material coated with an electrically conductive orsemi-conductive surface, and is provided with surfaces which will notgive rise to a powerful concentration of electric fields. The targetelectrode may also be constructed in a number of different, known ways,partly in dependence on the construction of the corona electrode. In theFIG. 2 embodiment the target electrode M is shown to comprise, by way ofexample, two mutually parallel plates located in the direction of theairflow duct. In the case of needle-shaped corona electrode the targetelectrode advantageously has the form of a cylinder arranged coaxiallywith the airflow duct. An electrically conductive surface coating on theinside of the airflow duct 1 may also serve as the target electrode. Thetarget electrode may also comprise a plurality of planar or cylindricalelectrode elements arranged in side-by-side relationship, with theirside surfaces substantially parallel with the longitudinal axis of theairflow duct 1. The target electrode may also comprise straight orhelically wound wires, or straight rods which may be arranged mutuallyparallel with one another or to cross one another to form a gridstructure, or may have the form of a perforated disc. A particularadvantage is afforded, however, when the target electrode has the formof an electrically conductive or semi-conductive surface which embracesthe airflow duct in the form of a frame and which has an extensionparallel with the airflow direction corresponding to at least one fifthof the distance between corona electrode and target electrode.

The aforedescribed exemplifying embodiments of the corona electrode andthe target electrode can, in principle, be used in all of theembodiments or arrangements according to the invention describedhereinafter.

In the arrangement illustrated in FIG. 2 the corona electrode K and thetarget electrode M are each connected in a conventional manner to arespective pole or terminal of a direct-current voltage source 3. In theillustrated example the corona electrode K is connected to the positiveterminal of the voltage source 3, so as to obtain a positive coronadischarge. In principle, however, the polarity of the voltage source 3may also be the opposite, so as to obtain a negative corona discharge. Apositive corona discharge is generally to be preferred, however, sinceless ozone, which is a poisonous gas, is produced with a positive coronadischarge than with a negative discharge.

In the arrangement illustrated in FIG. 2 the terminal of the voltagesource 3 connected to the corona electrode K is earthed, in accordancewith the invention, so that the potential of the corona electrode Kcoincides substantially with the potential of all other electricallyinactive parts of the actual arrangement similarly earthed, and alsowith the potential of the immediate surroundings of the arrangement. Thepotential of the corona electrode K will, in this way, be the same asthe potential of the environmental conditions located upstream of thecorona electrode K, with any electrically conductive objects or surfaceslocated in said environment, and hence no undesirable flow of ions willbe obtained from the corona electrode K in a direction upstreamtherefrom.

As mentioned in the aforegoing, the axial distance between the coronaelectrode K and that part of the target electrode M which receives thepredominant part of the ion current is at least 50 mm, and preferably atleast 80 mm, whereby air can be transported through the airflow duct ata throughput of, for example, 100 m³ /h with the aid of a low coronacurrent in the order of 20-50 μA, which is an acceptable value withrespect to the production of ozone and oxides of nitrogen. Further, aspreviously mentioned, an advantage is gained when the target electrode Mis connected to the d.c. voltage source 3 through a large limitingresistance 8, which in the event of a short circuit caused by touchingthe target electrode M limits the short circuiting current to a value ofat most about 300 μA. Since, as a result of its construction, the targetelectrode M has a not insignificant capacitance, it can suitably be madefrom a material of high resistivity. A suitable material in thisrespect, having a high resistivity and, at the same time, the requisiteability to conduct electricity, is a plastics material whichincorporates a finely divided electrically conductive material, such ascarbon black for example. Known materials of this kind from which targetelectrodes can be produced have a surface resistivity in the order of100 kΩ and more.

It will be understood from the aforegoing that an arrangementconstructed in accordance with the invention, for example in the mannerillustrated in FIG. 2, is quite safe to touch, and hence it is notnecessary to take any other safety measures or to provide any form ofsafety device in order to prevent intentional or unintentional contactwith either the corona electrode K or the target electrode M.Furthermore, since the corona electrode K is earthed, there is no riskof ion current flowing through any other location than the targetelectrode. When seen as a whole, this surprisingly enables, in reality,an air transporting arrangement according to the invention to beconstructed without including any form of air-flow duct 1 whatsoever, atleast when the primary purpose of the arrangement is to cause air tomove in the space or area in which the arrangement is installed. Forexample, an arrangement constructed in accordance with the invention mayhave the extremely simple form illustrated in FIG. 3. This embodiment ofthe arrangement according to the invention includes a corona electrode Kin the form of a wire stretched between holder means (shown solelyschematically) carried by suitable frame means (not shown in detail),and a target electrode M which is spaced from the corona electrode K andalso carried by the aforesaid frame means. The target electrode M maycomprise two mutually parallel, electrically conductive surfaces, whichalso lie parallel to the corona electrode K. Alternatively, the targetelectrode M may comprise a rectangular or circular frame-like electrodesurface whose axial extension coincides with the desired airflowdirection 2, as illustrated in the figure, this embodiment of the targetelectrode being the one preferred. It will be seen that in thisembodiment there is no airflow duct whatsoever surrounding the twoelectrodes K and M. As with the FIG. 2 embodiment, the corona electrodeK is connected to earth and to one terminal of the d.c. voltage source3, whereas the target electrode M is connected to the other terminal ofthe source 3 through a large ohmic resistance effective to limit a shortcircuiting current to an acceptable value, in the event of a shortcircuit created by contact with the target electrode M. The targetelectrode M is also formed from a material of high resistivity, so as tolimit the capacitive discharge current when contact is made with thetarget electrode. Tests carried out with an arrangement constructed inthe manner illustrated in FIG. 3 showed that the arrangement is able totransport air very effectively in the direction indicated by the arrow2, within the area embraced by the target electrode M. The testedarrangement incorporated a rectangular, frame-like target electrode Mhaving a cross-sectional area of 600×60 mm and an axial length of 25 mm.The distance of the target electrode from the corona electrode K was 100mm. A voltage of 25 kV was applied to the target electrode M, and thecorona current was 30 μA. The d.c. voltage source 3 had a terminalvoltage of 29 kV, and the series resistance 8 had a resistance of 132MΩ. This extremely simple arrangement resulted in an airflow of 60 m³ /hthrough the area enclosed by the target electrode M. When shortcircuiting the target electrode M of this arrangement, the shortcircuiting current was found to be only ≈220 μA, i.e. a current strengthwhich can hardly be felt should personal contact be made with the targetelectrode M. The arrangement is thus perfectly safe to touch, providedthat the actual voltage source 3 itself is electrically safe to touch.

As before mentioned, many cases are to be found in which it is notdesirable for the corona electrode to be connected to earth potential.In cases such as these, the requisite screening of the corona electrodein accordance with the invention can be achieved with an arrangement ofthe kind illustrated schematically and by way of example in FIG. 4. Inthis arrangement, the negative terminal of the d.c. voltage source 3,and therewith also the target electrode M, is connected to earth,whereas the corona electrode K is connected to the positive terminalthrough a large resistance effective to limit the short circuitingcurrent to an acceptable value in the event of a short circuit due tocontact with the corona electrode K. In order to prevent ions frommigrating upstream from the corona electrode K, a screen electrode S isarranged upstream of the corona electrode and connected thereto, so thatthe screen electrode S and the corona electrode K both have mutually thesame potential. The screen electrode S may have one of a number ofdifferent forms, depending upon the construction or form of the coronaelectrode used. When the corona electrode K comprises a thin, straightwire, the screen electrode may, for example, have the form of a rod or ahelically formed wire. The screen electrode may also comprise aplurality of rods or wires arranged in mutually parallel relationship orin a diamond configuration. The screen electrode S may also be in theform of a net or grid-like structure. Alternatively, the screenelectrode may comprise electrically conductive surfaces placed in theclose proximity of the wall of an airflow duct 1 or on the innersurfaces of said wall. In principle, the screen electrode S is given ageometric configuration and position relative to the corona electrode Ksuch that the screen electrode S forms an equipotential barrier orsurface which is impermeable to ions emanating from the corona electrodeK.

The screen electrode S need not necessarily be electrically connecteddirectly to the corona electrode K, but may also be connected to the oneterminal of a further d.c. voltage source 4, as schematicallyillustrated in FIG. 5, in a manner such that the screen electrode S hasthe same polarity as the corona electrode K in relation to the targetelectrode M, and preferably a potential which coincides substantiallywith the potential of the corona electrode K. The screen electrode S is,herewith, connected to the voltage source 4 through a large resistance 9effective to limit the short circuiting current in the event of contactwith the screen electrode 5.

It will be seen that in the case of an arrangement according to FIG. 5when the screen electrode S has a higher positive potential in relationto the target electrode M than the corona electrode K, the flow of ionsin a direction upstream from the corona electrode K is also effectivelyprevented hereby. Even though the screen electrode S might have asomewhat lower positive potential than the corona electrode K, so that asmall ion current is able to flow from the corona electrode to thescreen electrode S upstream thereof, this can be accepted provided thatthere is only a short distance between the corona electrode K and thescreen electrode S, so that the distance through which the ion currentmigrates in the upstream direction is very short, and therewith also theso-called current distance.

It will be understood that when the screen electrode S of the embodimentof FIG. 4 or FIG. 5 has a form, or construction, such as to present asignificant capacitance, the electrode is preferably made of a materialof high resistivity, so as to limit the capacitive discharge current toan acceptable level in the event of contact being made with theelectrode. This applies generally to all voltage carrying electrodesincorporated in an arrangement constructed in accordance with theinvention, when these electrodes have a not insignificant capacitance.The corona electrode, however, is normally always designed to have avery small capacitance, such as to be incapable of giving rise tosignificant capacitive discharge currents. Another generally applicablefeature is that all electrodes of an arrangement according to theinvention connected to a non-earthed terminal of a d.c. voltage sourceare preferably connected to said source through a resistance of suchhigh magnitude that in the event of a short circuit created by contactwith the electrode, the short circuiting current is limited to at most300 μA.

As mentioned in the aforegoing, requisite screening of the coronaelectrode against an undesirable flow of ions in the upstream directioncan also be achieved electrostatically, for example in the mannerillustrated in FIG. 6. In this embodiment, the airflow duct 1, the wallsof which consist of a dielectric material, such as a suitable plasticsmaterial, is extended through some considerable distance from the coronaelectrode K in the upstream direction. When the arrangement is in itsoperational mode there is produced on the walls of the duct 1 an excessof surface charges which generate an effective shield against the ioncloud in the vicinity of the corona electrode K, provided that the duct1 extends through a sufficient distance from the corona electrode insaid upstream direction. This effectively prevents the migration of anion current in a direction upstream of the corona electrode K. Theefficiency of the screen can be further improved, by dividing theairflow duct upstream of the corona electrode K into a plurality ofpart-ducts, with the aid of elongated partition walls, plates or strips7 made of a dielectric material, as schematically illustrated in FIG. 6.In order to provide an effective screen, the length of duct 1 locatedupstream of the corona electrode K should be at least equal to thedistance of the corona electrode from the target electrode M, andpreferably at least 1.5 times this distance. The length of duct requiredto provide an effective and efficient screen depends on the geometry ofthe airflow duct 1, and then primarily on its cross-sectionalconfiguration, and on whether or not dielectric partition walls 7 havebeen provided in the duct 1, upstream of the corona electrode 7. Whenseen generally, it will also be understood that the demands placed onthis screening of the corona electrode will depend upon the differencein potential between the corona electrode and the earthed surroundings;a smaller difference in these potentials will thus lessen the demandswhich need be placed on the screen.

When the corona electrode of an air transporting arrangement accordingto the present invention is effectively screened in one of the waysaforedescribed, such that substantially no ions will flow in theupstream direction from the corona electrode, the effectivetransportation of air through the arrangement is determined primarily bythe transport force generated by the ion current flowing from the coronaelectrode K to the target electrode M, and is proportional to theproduct of said ion current and the distance between the coronaelectrode and the target electrode.

An increase in the distance between the corona electrode K and thetarget electrode M, while simultaneously maintaining an unchanged ioncurrent between the electrodes, can be achieved by increasing thevoltage connected between the two electrodes, from the voltage source 3.Consequently, in accordance with the invention, there is advantageouslyapplied between the corona electrode and the target electrode adifference in potential of higher magnitude than has hitherto been usualin, for example, electrostatic filters or precipitators of the kind usedin domestic dwellings. It will be understood that when the potential ofthe corona electrode is increased relative to the surroundings, there isa still greater need to screen the corona electrode in the manneraforementioned. An increase in voltage, however, is also encumbered withan increase in the costs entailed, inter alia, by the high-voltageinsulation in both the actual voltage source itself and in the ion-windarrangement as such, and because of this there is naturally an upperlimit to which the voltage can be increased in practice. Oneadvantageous method of reducing these difficulties is to connect thecorona and target electrodes to potentials of opposite polarities inrelation to earth.

According to a further development of the invention it has provenpossible, however, to increase the distance between the corona electrodeK and the target electrode M substantially, and therewith the migrationdistance of the ion current, without any decisive reduction in thestrength of the ion current between these two electrodes and withoutneeding to increase the voltage level, by arranging a so-calledexcitation electrode E in the proximity of the corona electrode K, asillustrated by way of example in FIG. 7. In the exemplary embodiment ofFIG. 7, this excitation electrode E has the form of a rotationalsymmetrical ring E comprising an electrically conductive material, or atleast presenting a partially electrically conducting inner surface,which is arranged coaxially around the corona electrode K, which in thisembodiment has the form of a needle electrode. In view of the particularconfiguration of the corona electrode K of the illustrated embodiment,the target electrode M has the form of a cylinder arranged coaxially inthe duct, whereas the screen electrode S has the form of a ring arrangedcoaxially in relation to the corona electrode K and upstream thereof.Thus, the excitation electrode E is located at a shorter axial distancefrom the corona electrode K than the target electrode M and, in theillustrated embodiment, is connected to the same terminal of the d.c.voltage source 3 as the target electrode M, through a high ohmicresistance 6. The excitation electrode E thus adopts a potential havingthe same polarity as the potential of the target electrode M in relationto the corona electrode K. The potential difference between theexcitation electrode E and the corona electrode K, however, becomessmaller than the potential difference between the target electrode M andthe corona electrode K. The excitation electrode E contributes towardsgenerating a corona discharge and maintaining the same at the coronaelectrode K, even when the distance between the corona electrode K andthe target electrode M is increased without increasing the voltage ofthe voltage source 3 at the same time. Only a minor part of the coronaion-flow eminating from the corona electrode K will pass to theexcitation electrode E, while the major part of this corona flow orcurrent will still pass to the target electrode M and contribute intransporting air through the arrangement.

The effect produced by the excitation electrode E can be illustrated bythe diagram shown in FIG. 8, in which the curve A illustrates the coronacurrent I as a function of the voltage U between the corona electrodeand the target electrode in the absence of an excitation electrode. Aswill be seen, no corona discharge, and therewith corona ion-current,will take place at all until a given threshold voltage U_(T) isexceeded. On the other hand, when an excitation electrode is arrangedadjacent the corona electrode, the circumstances illustrated by thecurve B prevail, namely that a corona ion-current is initiated at a muchlower voltage with the axial distance between corona electrode andtarget electrode unchanged. Only a part of this corona ion-current willflow to the excitation electrode, whereas the remainder passes to thetarget electrode.

The excitation electrode together with the target electrode can also beconsidered as a two-part target electrode, whose one part is locatedclose to the corona electrode, when seen in the axial direction, andserves as an excitation electrode, while the other part is located at asubstantial axial distance from said corona electrode and serves as atarget electrode for that part of the corona ion-current providing themotive force for the air flow.

Consequently, an "excitation electrode" can be obtained, for example, inthe manner illustrated in FIG. 9, by extending a part of the targetelectrode M axially towards the corona electrode K, up to the proximityof said electrode or even beyond the same; the target electrode M inthis embodiment comprising a number of mutually parallel platesextending axially in the duct 1. In this case those parts of the targetelectrode M located axially nearest the corona electrode K function asan excitation electrode, although the major part of the coronaion-current will flow to that part of the target electrode locatedfurther away from the corona electrode in the axial direction, togenerate the desired ion-wind. When the excitation electrode E iscombined with the target electrode M in this manner, by extending thetarget electrode M axially to a location in the vicinity of the coronaelectrode, the target electrode may advantageously comprise a highlyresistive material or a highly resistive surface coating applied to theinner surface of a tube of insulating material, the distal end of thetarget electrode M in relation to the corona electrode K being connectedto one terminal of the d.c. voltage source 3. That part of the targetelectrode located nearest the corona electrode K in the axial directionwill therewith serve as an excitation electrode E, which receives only aminor part of the corona ion-flow. Alternatively, a combined target andexcitation electrode can be obtained by providing the target electrode Mwith parts which extend axially towards the corona electrode K and up tothe vicinity thereof, and which exhibit a much smaller electricallyconductive area than the major part of the target electrode M locatedfurther away from the corona electrode K and connected to one terminalof the d.c. source. Those parts of the target electrode of smallconducting area located axially in the proximity of the corona electrodeK will thus serve as an excitation electrode, to which only a minor partof the total corona ion-flow deriving from the corona electrode K willpass.

The excitation electrode can be formed and arranged in many differentways. Any form of electrode which is located in the axial proximity ofthe corona electrode K and which does not in itself produce a coronadischarge and which is connected to one terminal of a direct-currentvoltage source, the other terminal of which is connected to the coronaelectrode, is able to serve as an excitation electrode, if only a minorpart of the total corona ion-current flows to this excitation electrodewhile the larger part of the corona ion-current flows to the targetelectrode. Thus, a screen electrode located upstream of the coronaelectrode and arranged to receive a given, small ion-current, forexample in accordance with the embodiment of FIG. 5, is able to functionas an excitation electrode.

The geometric form of the excitation electrode E may also vary independence on the configuration of the corona electrode K. For example,when the corona electrode comprises a plurality of geometricallyseparated but electrically connected electrode elements, for examplestraight thin wires arranged side-by-side, the excitation electrode mayadvantageously also comprise a plurality of geometrically separated butelectrically connected electrode elements, which are then arrangedbetween the electrode elements of the corona electrode so as to bescreened from each other, which in respect of such a corona electrode isadvantageous to the creation of the corona ion-current.

FIG. 9 illustrates schematically and by way of example an arrangementaccording to the invention which incorporates a corona electrode K, atarget electrode M, a screen electrode S and an excitation electrode E.In this embodiment each electrode comprises a plurality of geometricallyseparated but electrically connected electrode elements, which in thecase of the corona electrode K comprise straight, thin wires made oftungsten for example, whereas the other electrodes comprise helicallyformed wires of, for example, stainless steel.

Since, as evident from the aforegoing, an arrangement according to theinvention can be readily constructed so that all electrodes are safe totouch, it will be understood that the embodiments illustrated, forexample, in FIGS. 4, 5, 7, 9 and 10, in which the target electrode M isearthed and the corona electrode K and the screen electrode and alsooptionally the excitation electrode E are connected to a higherpotential, can also be constructed to exclude an airflow duct whichsurrounds the electrodes, provided that the screen electrode isconstructed in a manner which ensures that it will effectively preventthe ion current eminating from the corona electrode from flowing in anyother direction than towards the target electrode.

Although an arrangement according to the invention is able to functionquite satisfactorily in the absence of any form of airflow duct aroundthe electrodes of the arrangements, the provision of such a duct may bedesirable in some instances, however, for example for psychologicalreasons or because such a duct will conduct the air through thearrangement in a more orderly fashion. The provision of such a duct mayalso be unavoidable in some instances, for example when the arrangementis to be placed within a ventilation duct in a ventilation system, or inother instances where the airstream generated by the arrangement is tobe conducted from and/to specific locations. The presence of such anairflow duct which encloses the electrodes of the arrangement and thewalls of which, quite naturally, consist of an electrically insulatingmaterial, gives rise to troublesome problems however. As discussed abovewith reference to FIG. 6, there appears on the inner surfaces of thewall of such a duct an excess of electrical surface charges. A similarexcess of surface charges will naturally also appear on that part of theduct wall located between the corona electrode and the target electrode,and will influence the desired ion-current flowing from the coronaelectrode downstream towards the target electrode, in a manner such asto tend to restrict the ion-current to the central region of thecross-sectional area of the air-flow duct, which results in an unevendistribution of the airflow across the width of the duct, therewithimpairing transportation of air therethrough. This problem is greatlyexacerbated by variations in the voltage applied to the corona electrodeand the target electrode through the aforesaid voltage source. A temporyincrease in the voltage will namely result in an increase in theaforesaid surface charges, these charges persisting even when thevoltage is subsequently lowered, and therewith cause a strong reductionin the corona current and therewith in the transporation of air throughthe arrangement. The drawbacks created by this phenomenon can beovercome, or at least greatly alleviated, by stabilizing the voltagedelivered by the voltage source, this expedient being of no particularinterest from other aspects in an arrangement of the kind in question,or by briefly cutting-off the voltage to the electrodes at uniformlyspaced time intervals. The excess surface charges present on the innersurfaces of the duct wall namely disappear relatively quickly when thevoltage supply is interrupted and the electric field thereby removed.The presence of excess electrical charges on the inner surfaces of theelectrically insulating duct wall give rise, however, to an additional,highly surprising and serious problem. It has namely been found thatwhen the inner surface of the insulating duct-wall is touched, evenbriefly, the flow of corona current will cease totally, and is notautomatically stored, not even after the lapse of a very long period oftime from when the surface was touched. Obviously, a solution to thisproblem must be found.

One possible solution to this problem is to apply an electricallyconductive layer to the outer surface of the insulating wall of the ductand to earth said layer. However, this would give a high capacitance toa target electrode located in the close proximity of the duct wall, orlocated directly on the inner surface of said wall, which as mentionedin the aforegoing is undesirable with respect to the safe-to-touchaspect of the target electrode. It has been found possible to avoidthis, however, by increasing the cross-sectional dimensions of theairflow duct to a size substantially greater than the correspondingdimensions of the area enclosed by the target electrode, so that thetarget electrode is located at a substantial distance from the innersurface of the airflow duct. One such embodiment is illustratedschematically in FIG. 11. In this embodiment, the outer surface of theinsulating wall of the duct 1 is provided with an electricallyconductive layer 10, which is earthed. The duct 1 of this embodiment isalso significantly wider than the target electrode M, so that the ductwalls are further away from the target electrode, which thereby obtainsa much lower capacitance. The duct walls have, in this way, also beenplaced further away from the corona electrode K, and hence the excesscharges occurring on the inner surface of the insulating duct-wall havea much less disturbing effect on the corona current flowing from thecorona electrode K to the target electrode M. This increase in thecross-sectional dimensions of the airflow duct 1 in relation to thecross-sectional dimensions of the target electrode M has not been foundto have any deleterious effect on the transporation of air through thearrangement, but that in fact such transportation is increased at anunchanged corona current. In the embodiment illustrated in FIG. 11, thecentre point of the d.c. voltage source 3 is earthed, so that the targetelectrode M and the corona electrode K have opposite polarities inrelation to earth, which restricts the total high-voltage level requiredand therewith the necessity to insulate the arrangement against highvoltages, and also reduces the demands on the screening of the coronaelectrode K, as mentioned in the aforegoing. Since, in this case, a highvoltage is applied to the screen electrode, the corona electrode and thetarget electrode, all of the said electrodes are connected to the d.c.voltage source through a large resistance 8 effective to limit the shortcircuiting current in the event of contact with the electrodes.Moreover, both the target electrode M and the screen electrode 7 aresuitably manufactured from a material of high resistivity, in order tolimit the capacitive discharge current in the event of contact.

In an embodiment of this kind, an advantage is gained when thecross-sectional dimensions of the airflow duct 1 are adapted so that thedistance between the duct wall and corona electrode K is equal toapproximately half the distance between the corona electrode and targetelectrode, and so that the distance between duct wall and the surface ofthe target electrode is approximately 50% of the cross-sectionaldimension of the target-electrode aperture.

The aforedescribed unfavourable effects caused by the presence of excesscharges on the inner surface of the duct wall can also be reduced withthe aid of an excitation electrode having the function described in theaforegoing, this excitation electrode comprising an electricallyconductive layer applied to the inner surface of the duct wall. As willbe understood, no excess charges are able to appear on the inner surfaceof the duct wall in the presence of such an excitation electrode. If, inthis respect, the cross-sectional dimensions of the airflow duct areincreased to an extent such that the target electrode is located at asignificant distance from the wall of the duct, as illustrated in FIG.11 and described above, the excitation electrode mounted on the innersurface of the duct wall can be very surprisingly extended in thedownstream direction, to a location beyond the target electrode. Inactual fact, in this particular case an electrically conductive layercan be provided on the inner surface of the duct wall throughout thewhole length of the duct, i.e. even in the upstream direction to alocation beyond the corona electrode. One such embodiment is illustratedschematically in FIG. 12.

Thus, the embodiment illustrated in FIG. 12 includes an airflow duct 1,the wall of which is assumed to consist of an electrically insulatingmaterial and the inner surface of which is provided with an electricallyconductive coating E, which is earthed and which functions as anexcitation electrode in the vicinity of the corona electrode K. Thecross-sectional dimensions of the duct 1 are such that a targetelectrode M, of frame-like configuration and extending parallel with thewalls of the duct 1, is located at a significant distance from the innersurface of the duct wall, and is thus well insulated from theelectrically conductive coating E on the inner surface of the duct wall.Located upstream of the corona electrode K is a number of screenelectrodes S, for example in the form of coarse rods. The d.c. voltagesource is earthed at its central point, so that the corona electrode Kand the target electrode M have opposite polarities in relation toearth, which affords the aforedescribed advantages. The electrodes arealso connected to the d.c. voltage source through large resistances 8,to limit the short circuiting current. It will be seen that no excesssurface charges whatsoever can appear on the inner surface of the ductwall in an embodiment of the arrangement such as this, and hence thearrangement is not encumbered with those problems arising from thepresence of such excess surface charges. This embodiment of anarrangement according to the invention has also been found to transportair in an exceedingly satisfactory manner. The conditions mentionedabove with reference to FIG. 11 also apply with regard to thedimensioning of the airflow duct 1 of the FIG. 12 embodiment.

It will be understood that since it is possible with an arrangement suchas that illustrated in FIG. 12 to provide the inner surface of the ductwall with an electrically conductive, earthed coating along the wholelength of the duct, there is nothing to prevent the duct wall fromconsisting entirely of an electrically conductive material, which wouldnaturally facilitate manufacture considerably, and also afford othervaluable advantages. Thus, it is possible that the inner surface of theduct be lined, at least along a given part of its length, with achemically adsorbing or absorbing material, for example a carbon filter,effective to remove gaseous contaminents from the air, such as odoursand the oxides of nitrogen generated by the corona discharge, byabsorption or adsorption. It is also possible, for the same purpose, topass a thin liquid film, for example water or a chemically activeliquid, along the inner surface of the airflow duct. The wall of theair-flow duct can also be cooled or heated, with the aid of suitablemeans, for example circulating water, in order to cool or heat thetransported air. All this is made possible by the fact that the wall ofthe airflow duct is electrically conductive and earthed.

In those embodiments of the arrangement according to the invention inwhich the electrodes are enclosed in an airflow duct it has been foundto be advantageous to use one single corona electrode K arrangedcentrally therein, since the greatest possible distance between the ductwall and the corona electrode is obtained in this way, and therewith theleast possible disturbance in the function of the corona electrode as aresult of the duct wall. Alternatively, there can be used, however, twocorona electrodes placed symmetrically on a respective side of thesymmetry plane of the duct. In this arrangement each electrode will beaffected solely by one wall or side of the duct and both electrodes willoperate under mutually similar conditions. This does not apply, however,when more than two electrodes are installed in the duct. In thoseembodiments where two corona electrodes are placed symmetrically in theairflow duct, it can be to advantage to also install two targetelectrodes side-by-side in a similar symmetrical relationship, thetarget electrodes in this respect suitably having a common electricallyconducting wall.

In the case of an embodiment such as that illustrated in FIG. 12, itwill be understood that the electrically conductive and earthed coatingor lining E on the inside of the insulating airflow duct 1 need not beextended upstream of the corona electrode K, in which case the excesscharges consequently appearing on the inner surface of the electricallyconductive duct wall upstream of the corona electrode K will co-operatein establishing the necessary screening of the corona electrode K.

A further problem, affecting the total transportation of air through anarrangement of this kind, occurs when the corona electrode has the formof a wire extending across the path of the airflow and attached at bothends to electrically insulated attachment means. The same problem canalso occur with other types of electrode which extend across the path ofthe airflow. In this respect it has been found that the corona electrodegives much more corona current per unit of length within the centralregion of the airflow path than at the end parts of the electrode. Thiswould appear to be due to a screening effect created through theelectrode attachment means and through the wall of the duct at both endsof the electrode, when an airflow duct is included in the arrangement.In the case of a low corona current, a considerable part of both ends ofthe corona electrode can even be "extinguished" or cut-out. This resultsin uneven distribution of the ion current and therewith unevendistribution of the airflow across the cross-sectional area of the pathtaken by the airflow. When the arrangement incorporates an airflow ductwhich surrounds the electrodes, it has been found that when seen incross-section, those parts of the airflow duct located oppositerespective ends of the corona electrode exhibit an airflow which movesin a direction opposite to that intended. This phenomenon can greatlyimpair, and even totally eliminate effective transportation of airthrough the arrangement. This problem can be overcome, however, inaccordance with a further development of the invention, by giving thetarget electrode and/or the excitation electrode a particular form. Anembodiment of a target electrode suitably formed in this latter respectis illustrated schematically and by way of example in FIG. 3, whichshows an arrangement according to the invention, incorporating anairflow duct 1, shown in broken lines, of narrow, elongated rectangularcross-section. Extending across the duct 1, between the two short wallsthereof, is a wire-like corona electrode K. The target electrode M hasthe form of a conductive layer or coating on the inner surfaces of theduct wall and, in this embodiment, is so formed that when seen in theaxial direction of the duct it lies closer to the end portions of thecorona electrode K than to the central region of said corona electrodein the transverse direction of the duct. For example, the axial distancebetween the target electrode M and the corona electrode K at the centreregion thereof may be 60 mm, while the corresponding axial distance fromthe target electrode to the opposite located end portions of the coronaelectrode is only 40 mm. A target electrode M of this configuration willeliminate the problem discussed above, so as to obtain substantiallyuniform distribution of the corona current along the whole length of thecorona electrode.

The same result can be achieved when an excitation electrode arrangedbetween the corona electrode K and the target electrode M is formed inthe manner described above with reference to FIG. 13 in respect of thetarget electrode. In this case the target electrode can either be formedin the manner illustrated in FIG. 13 or in a normal manner, i.e. so thatits axial distance from the corona electrode is the same at all pointsthereon. A corresponding result can also be obtained with the aid ofexcitation electrodes which are located solely in the vicinity of bothend portions of the corona electrode. A most essential feature, however,is that the target electrode and/or the excitation electrodes is, orare, so formed that the corona electrode K extending across the airflowpath provides substantially the same amount of corona current per unitlength over the whole of its length, i.e. even at the end portions ofthe corona electrode.

A target electrode and excitation electrode having the form describedwith reference to FIG. 12 may also be used to advantage in anarrangement in which the electrodes are not enclosed in an airflow duct,since a target electrode and excitation electrode thus formed willenable the corona current to be distributed more uniformly over thewhole length of the electrode.

An arrangement according to the invention and constructed in accordancewith the embodiment illustrated in FIG. 10 was used in practice forexperimental purposes. In this experimental arrangement, the distancebetween the plane of the screen electrode S and the plane of the coronaelectrode K was 12 mm, whereas the distance between the plane of thecorona electrode K and the target electrode M was 85 mm. The mutualdistance between the wire-like electrode elements in the coronaelectrode K was 50 mm, and the electrode element of the excitationelectrode E was arranged in the same plane as the electrode elements ofthe corona electrode K centrally therebetween. The various electrodeswere connected to the voltages given in the drawings. The airflow duct 1measured 35×22 cm in cross-section, and an earthed protective grid G wasarranged at the inlet to the duct. When this apparatus was placed freelyon a table, an airflow velocity in excess of 0.5 m/s was obtained. Thetotal corona current from the corona electrode K was about 50 μA, ofwhich about 40 μA passed to the target electrode M. An airflow velocityof about 0.5 m/s was obtained at a power consumption of 5-6 W/m² of thearea of the flow duct. The power required to obtain a correspondingairflow velocity in a similar apparatus lacking the screen electrode Sand the excitation electrode E but with the same voltage on the coronaelectrode was about 100 W/m². In this case, the distance between thecorona electrode K and the target electrode M was about 50 mm, and thedistance between the corona electrode K and the protective grid G at theduct inlet was 100 mm. In this embodiment of the apparatus according tothe invention, the distance of the protective grid G from the coronaelectrode K had no noticable influence on the efficiency of theapparatus.

The transportation of air through an arrangement, or apparatus,constructed in accordance with the invention can be further increased byarranging a plurality of electrode arrays, each array comprising acorona electrode, target electrode, screen electrode and optionally anexcitation electrode, sequentially in one and the same airflow duct. Thearrangement of a screen electrode upstream of each corona electrode, inthe aforedescribed manner, will effectively prevent the undesirable andharmful flow of ions in the upstream direction, such flow beingunavoidable in such a cascade arrangement in the absence of a screenelectrode.

The arrangement provides an extremely effective air transportingarrangement of relatively simple construction. In addition, anarrangement constructed in accordance with the invention is relativelyinexpensive, and has small dimensions and a low weight. Such anarrangement also has a low energy consumption and is absolutely silentin operation.

When an air transporting arrangement according to the invention is usedin conjunction with an electrostatic filter device, the target electrodeM in the air transporting arrangement can be arranged to formsimultaneously parts of the precipitation surfaces incorporated in theelectrostatic filter arrangement for receiving the impurities chargedupon collision with the air ions, for example in a capacitor separatorof a kind known per se. When the target electrode M functions as aprecipitation surface for impurities carried by the air transportedthrough the arrangement, the target electrode is suitably constructed ina manner which enables it to be readily dismantled for replacement orcleaning purposes when the electrode becomes excessively coated withprecipitated contaminents. It will be seen that this can be readilyachieved when the arrangement does not incorporate an airflow ductsurrounding the electrodes. In contexts such as these the targetelectrode can conceivably have the form of strip material fed from astorage reel or fed through a cleansing device when the part of thestrip material used as a target electrode has been dirtied byprecipitated contaminents.

We claim:
 1. An apparatus for transporting air with the aid of anelectric ion-wind, comprising at least one corona electrode and at leastone target electrode which is permeable to an airflow through theapparatus and which is located at a distance from and downstream of thecorona electrode, as seen in the direction of said airflow; a d.c.voltage source having one terminal thereof connected to the coronaelectrode and the other terminal thereof connected to the targetelectrode, the construction of the corona electrode and the voltagebetween the terminals of the voltage source being such that a coronadischarge generating air ions occurs at the corona electrode; andscreening means for screening the corona electrode in a directionupstream of said corona electrode, such that the product of the value ofany ion current in said upstream direction and the distance migrated bysaid any ion current from the corona electrode is practically zero, orin all events much smaller than the product of the value and themigration distance of the ion current in a direction downstream from thecorona electrode to the target electrode; and the distance between thecorona electrode and the part of the target electrode receiving thepredominant part of said downstream ion current being at least 50 mm. 2.An apparatus as claimed in claim 1, wherein the distance between thecorona electrode and the part of the target electrode receiving thepredominant part of the downstream ion current is at least 80 mm.
 3. Anapparatus as claimed in claim 1, wherein said screening means includeany electric connection between the corona electrode and groundpotential.
 4. An apparatus as claimed in claim 1, wherein said screeningmeans include an electrically conductive screen electrode locatedupstream of the corona electrode and having a potential of the samepolarity in relation to the target electrode as the potential of thecorona electrode.
 5. An apparatus as claimed in claim 4, wherein thescreen electrode is electrically connected to the corona electrode. 6.An apparatus as claimed in claim 1, wherein said screening means includean airflow duct enclosing at least the corona electrode and having wallsconsisting of a dielectric material, which walls are extended upstreamof the corona electrode through a distance which is at least equal tothe distance between the corona electrode and the target electrode. 7.An apparatus as claimed in claim 6, wherein the walls of said airflowduct are extended upstream of the corona electrode through a distancewhich is at least 1.5 times the distance between the corona electrodeand the target electrode.
 8. An apparatus as claimed in claim 6, whereinsaid airflow duct upstream of the corona electrode is provided withpartition walls made of a dielectric material and extendingsubstantially parallel to the longitudinal extension of the duct.
 9. Anapparatus as claimed in claim 1, comprising an excitation electrodelocated in the vicinity of the corona electrode at a shorter axialdistance therefrom than the target electrode; said excitation electrodebeing connected to a potential of the same polarity relative to thecorona electrode as the potential of the target electrode to co-operatein the generation of the corona discharge at the corona electrodewithout giving rise to a corona discharge at itself, the part of thetotal ion current passing from the corona electrode to the excitationelectrode being substantially smaller than that part of said total ioncurrent passing to the target electrode.
 10. An apparatus as claimed inclaim 9, wherein the potential difference between the excitationelectrode and the corona electrode is smaller than the potentialdifference between the target electrode and the corona electrode.
 11. Anapparatus as claimed in claim 10, wherein the excitation electrode isconnected to the same terminal of the d.c. voltage source as the targetelectrode through a large resistance.
 12. An apparatus as claimed inclaim 1, wherein the target electrode is extended towards the coronaelectrode up to the axial proximity at the corona electrode, theelectrically conductive material of the target electrode has a highresistivity and said other terminal of the d.c. voltage source isconnected to the part of the target electrode located furthest away fromthe corona electrode, whereby said part of the downstream ion currentfrom the corona electrode and the part of the target electrode locatedin the axial proximity of the corona electrode is functioning as anexcitation electrode assisting the generation of the corona discharge atthe corona electrode.
 13. An apparatus as claimed in claim 1, whereinthe target electrode is provided with electrically conductive partsextending axially towards the corona electrode up to the axial proximityof the corona electrode and having a substantially smaller electricallyconductive area than the major part of the target electrode located at asubstantial axial distance from the corona electrode, said major partbeing connected to said other terminal of the d.c. voltage source toreceive the predominant part of the downstream ion current from thecorona electrode, and said parts located in the axial proximity of thecorona electrode functioning as an excitation electrode assisting thegeneration of the corona discharge at the corona electrode.
 14. Anapparatus as claimed in claim 1, wherein the target electrode compriseselectrically conductive surfaces which extend parallel with thedirection of airflow and enclose the airflow path.
 15. An apparatus asclaimed in claim 1, wherein the electrodes are arranged within anairflow duct and the target electrode comprises electrically conductivesurfaces on the wall of the airflow duct.
 16. An apparatus as claimed inclaim 1, wherein the electrodes are arranged within an airflow duct, thetarget electrode comprises electrically conductive surfaces which extendparallel with the wall of the airflow duct and are located at a distanceinwardly thereof; and the wall of said airflow duct compriseselectrically insulating material and has located externally thereof anearthed electrically conductive surface.
 17. An apparatus as claimed inclaim 1, wherein the electrodes are arranged within an airflow ducthaving a wall having at least one electrically conductive inner surfacewhich is earthed; the target electrode comprises electrically conductivesurfaces which are parallel with the wall of the airflow duct andlocated at a substantial distance inwardly thereof; and the targetelectrode and the corona electrode are connected to potentials ofopposite polarities in relation to earth.
 18. An apparatus as claimed inclaim 17, wherein the wall of the airflow duct is electricallyconductive in its entirety.
 19. An apparatus as claimed in claim 17,wherein the airflow duct has a wall which consists of an electricallyinsulating material and which is provided on the inner surface thereofwith an electrically conducting layer which extends axiallyapproximately from the corona electrode to a location downstream of thetarget electrode.
 20. An apparatus as claimed in claim 16, wherein thedistance between the wall of the airflow duct and the nearest lyingsurface of the target electrode corresponds approximately to 50% of thecross-section dimension of the area surrounded by the target electrode.21. An apparatus as claimed in claim 17, wherein at least a part of theinner surface of the airflow duct is provided with a layer of chemicallyabsorbing material.
 22. An apparatus as claimed in claim 17, wherein atleast part of the inner surface of the airflow duct is flushed withwater or a chemically active liquid.
 23. An apparatus as claimed inclaim 17, comprising means for controlling the temperature of the ductwall.
 24. An apparatus as claimed in claim 1, wherein electrodes havinga high potential in relation to earth are connected to the d.c. voltagesource through resistances of such high resistance value, that in theevent of any of said electrodes being earthed the resultant shortcircuiting current will reach at most approximately 300 μA.
 25. Anapparatus as claimed in claim 1, wherein electrodes having a potentialwhich differs from each potential and a substantial capacitance comprisea material of high resistivity, so that in the event of contact with anyof said electrodes the capacitive discharge current will be limited toan acceptable value.
 26. An apparatus as claimed in claim 1, wherein thecorona electrode and the target electrode are connected to potentials ofopposite polarities in relation to earth.
 27. An apparatus as claimed inclaim 1, wherein the corona electrode extends transversely across theairflow path; the target electrode comprises an electrically conductivesurface which embraces said path and extends parallel thereto; and theaxial distance between the corona electrode and the nearest edge of theconductive surface of said target electrode is shorter at locationsopposite the end portions of the corona electrode than at locationsopposite the center region of said corona electrode.
 28. An apparatus asclaimed in claim 9, wherein the corona electrode extends transverselyacross the airflow path; the excitation electrode comprises anelectrically conductive surface embracing said airflow path andextending parallel therewith; and the axial distance between the coronaelectrode and the nearest edge of the conductive surface of theexcitation electrode is shorter at locations opposite the end portionsof the corona electrode than at locations opposite the central region ofsaid corona electrode.
 29. An apparatus as claimed in claim 9, whereinthe corona electrode extends transversely across the airflow path; theexcitation electrode comprises electrically conductive surfacesextending parallel with the airflow path; and the electricallyconductive surfaces forming said excitation electrode are locatedsubstantially axially opposite the end parts of the corona electrode.